1. Field of the Invention
The invention relates to an implantable catheter assembly for hemodialysis. More particularly, the invention relates to an implantable catheter assembly providing long or short term access via a needle wherein a puncture region is readily sealed after removal of the puncturing needle.
2. Description of the Prior Art
Patients with renal failure face certain death unless managed by either hemodialysis or peritoneal dialysis. Renal transplantation is the best, albeit less available, solution to this problem. As a result, various dialysis solutions have been developed. One current hemodialysis solution includes the surgical creation of an arterial to venous fistula using either native vein or polyfluorotetraethylene (PTFE) prosthetic vein. However, fistulas such as these are prone to clotting and have an average life span of only 1 to 1½ years.
A recent alternative to surgically placed grafts is a fluoroscopically placed hemodialysis catheter, or vascular access device. Vascular access devices are placed in an outpatient setting, can remain in place for months, and provide excellent flow rates necessary for hemodialysis. Unfortunately, infection, skin loosening, and the possibility of catheters coming apart have resulted in great expense, as well as patient morbidity and mortality.
Such vascular access devices are disclosed in prior art. For example, attention is directed to the devices disclosed in U.S. Pat. No. 4,692,146 to Hilger, U.S. Pat. No. 5,041,098 to Loiterman et al., U.S. Pat. No. 5,848,989 to Villani and U.S. Pat. No. 5,944,688 to Lois. In general, Loiterman discloses penetrating a “resealable” elastomeric septum with a large diameter needle. However, Loiterman does not mention how the resealing is accomplished.
Experiments have shown that repetitive punctures of a septum with large diameter needles will result in the leakage of blood through the septum. With this in mind, nowhere does Loiterman discuss how to achieve a septum seal after repetitive punctures with a large diameter needle. Loiterman merely discloses the septum as comprising a self-sealing material. However, it is highly unlikely that the structure disclosed by Loiterman would in fact work as proposed for an extended period of time.
Lois, like Loiterman, only refers to the disclosed septum as being constructed of a “self-sealing material”. Lois provides no details as to making certain the sealing is accomplished after multiple punctures with a large bore needle.
Further, and with reference to Villani and Hilger, they both provide an elastomeric septum having a similar design. Generally, a cylindrical elastomeric plug is compressed axially along a radial portion of the plug. This compresses the plug axially along a small radial portion at the outside diameter of the plug, producing convex bulges at both ends of the plug. In this way, both Villani and Hilger attempt to provide sealing of the plug by axial compression while simultaneously providing needle access through the plug. A finite element analysis (see FIGS. 1 to 10) of the Villani and Hilger septums clearly shows that repetitive needle punctures in a septum of this design will not seal. That is, the stress distribution within the septum tends to keep the hole open, thereby permitting the passage of blood through the open hole (which is highly undesirable).
The finite element analysis of the Villani and Hilger type designs was conducted to determine whether or not sealing is obtained once a hole is produced in the septum. The criterion upon which sealing is determined is the deformed geometry of the plug in the region of the hole. As those skilled in the art will well appreciate, finite element analysis is a universally accepted method for conducting structural analysis. It enables one skilled in the art to determine the deformation and stress distributions in an engineered product. With this in mind, finite element analysis of the designs disclosed by Villani and Hilger was conducted to evaluate their sealing capabilities. The results of the study show that it is possible to produce holes in the septums disclosed by Villani and Hilger which will not seal, that is, holes in the septum will remain open permitting the passage of blood through the septum. In fact, five studies were conducted. Four models were asymmetrical and the fifth model was three dimensional.
FIG. 1 shows an asymmetrical model of the Villani/Hilger design. The model is symmetrical about the center line and has a 0.002″ radial hole in the septum.
FIG. 2 shows the deformed geometry of the septum when loaded as per the Villani/Hilger patents. Note that the analysis correctly shows the convex geometry produced in the septum as a result of the loading. The model clearly shows that the hole does not close. FIG. 2A is a close up of the central portion of the model shown in FIG. 2. The Figure shows the deformed geometry of the model along with the center line and the undeformed 0.002″ radius of the hole. The close up in FIG. 2A shows the open hole as the distance between the central line and the upper edge of the deformed model. The graph in FIG. 3 shows the radial displacement of the entire length of the hole in the deformed septum. The undeformed hole radius is 0.002″. The two ends of the graph show that a 0.002″ deformed hole at the ends of the septum will only close by 0.00025″, i.e., the radius of the open hole at the ends of the septum is 0.002″−0.00025″=0.00175″.
In a similar fashion, the radius of the hole may also be determined through the entire length of the septum. Note that the “distance” scale on the graph represents the fraction of the distance along the length of the septum. Zero and 1 are the two ends of the septum; 0.25 is a point ¼ of the distance from the left end of the septum, etc. Note that the greatest closure of the 0.002″ hole occurs at distances of 0.25 and 0.75. At these two locations, a hole of radius 0.002″−0.00182″=0.00018″ still exists. The study, therefore, shows that a hole in the septum with a radius of 0.002″ will remain open with a radius value ranging from approximately 0.00018″ to 0.00175″.
The study was repeated for initial hole diameters of 0.001″, 0.0005″ and 0.0001″. The results are shown in FIGS. 4, 5 and 6. Note that even if the initial hole in the septum has a radius of 0.0001″, the hole in the deformed septum remains open with a radius of at least 0.000041″.
A three-dimensional FEA model of the Villani/Hilger septum was then studied for verification purposes. The undeformed model is shown in FIG. 7. The model has a central hole of radius 0.002″. The deformed geometry of the septum is shown in FIGS. 8 and 9. Note that the model clearly shows the convex geometry at the ends of the septum in the deformed state. FIG. 10 is a view along the Z-axis of the deformed septum, i.e., looking directly down the deformed hole. From FIG. 10 we see that the hole closes slightly at the ends of the septum FIG. 10 also clearly shows an open hole completely through the septum through which undesirable blood flow may exist. The three-dimensional model results verify the two-dimensional model results.
Villani and Hilger attempt to seal the needle puncture holes by compressing the septum axially. The finite element analysis of the Villani/Hilger design shows that the septum cannot be effectively sealed using this design. Once a hole has been established in the septum by repeated needle punctures, the hole remains open and will permit an undesirable blood flow passage through the septum.
In summary, the present invention provides a resealable vascular access device substantially distinct from those disclosed in the prior art. With this in mind, the present invention overcomes the prior art by providing an implantable catheter device resealable so as to permit multiple punctures by large diameter needles.